Reduce boilerplate in compiler JUnit tests (#5158)

Many JUnit tests share a compiler instance between all
test cases in a class to reduce overhead.

This commit refactors the mechanism to reduce the boilerplate.

In the new scheme:

  - Using the `@ClassRule` hook in JUnit, we create a per-class
    map for each test class.
  - Per-class values are registered from the test class itself
    by calling `cached("someKey", () => mkExpensiveThing)`
  - At the end of the test, the entries in this map are `close()`-ed
    (if they implement `Closable`), and are released for
    garbage collection.)

1 files changed, 1 insertions, 7 deletions

diff --git a/test/junit/scala/tools/nsc/backend/jvm/StringConcatTest.scala b/test/junit/scala/tools/nsc/backend/jvm/StringConcatTest.scala
index 2a9b8f7198..fc0c96e71a 100644
--- a/test/junit/scala/tools/nsc/backend/jvm/StringConcatTest.scala
+++ b/test/junit/scala/tools/nsc/backend/jvm/StringConcatTest.scala
@@ -14,15 +14,9 @@ import scala.tools.partest.ASMConverters
 import ASMConverters._
 import scala.tools.testing.ClearAfterClass
 
-object StringConcatTest extends ClearAfterClass.Clearable {
-  var compiler = newCompiler()
-  def clear(): Unit = { compiler = null }
-}
-
 @RunWith(classOf[JUnit4])
 class StringConcatTest extends ClearAfterClass {
-  ClearAfterClass.stateToClear = StringConcatTest
-  val compiler = StringConcatTest.compiler
+  val compiler = cached("compiler", () => newCompiler())
 
   @Test
   def appendOverloadNoBoxing(): Unit = {